Thermal and Operational Performance of Gravity Loop Heat Pipes with Flat and Finned-Tube Evaporators in a Gas Fireplace Insert

Evaporator designs of passive heat pipes for combustion air preheating in local space heaters have not been compared directly. This study compares two complete evaporator designs of a gravity loop heat pipe in a gas fireplace insert: a flat evaporator with a glass-fibre wick and a copper finned-tube evaporator. Two test series at four settings (4.5–9.6 kW) were compared with a reference without the heat pipe. The heat pipe transferred 953–2040 W to the combustion air (point velocity upper estimate; 523–1202 W from the excess air ratio), preheating it to 50–85 °C. The EN 613 efficiency rose by 1.5–5.7 percentage points at the combustion chamber boundary but changed by only −1.1 to +1.2 for the whole insert, as the evaporator also absorbs firebox radiation and preheating heat largely circulates internally. The finned-tube evaporator started up faster (10–29 vs. 33–42 min), but its series differed by up to 25%. At the highest settings, the flat evaporator lowered the flue gas temperature by 14–15 K but raised the glass temperature by 46–59 K. Loop pressures up to 6.6 bar limited operation. The evaporator design thus mainly affected start-up and firebox and flue gas temperatures; whole-appliance efficiency gains require lower flue gas temperatures.